[Twisted-web] Deferred in proxy.ProxyClient.handleHeader (reformatted)

exarkun at twistedmatrix.com exarkun at twistedmatrix.com
Fri May 28 15:09:11 EDT 2010

On 12:47 pm, stefan at grosz.at wrote:
>(sorry, the formatting in my last mail got screwed up. hope this time 
>it works)
>i'm currently writing a reverse proxy with twisted and i've a problem 
>with a deferred function in my handleHeader method.
>the code looks simplified like this (http://pastie.org/981735):
>class MyProxyClient(proxy.ProxyClient):
>    @defer.inlineCallbacks
>    def handleHeader(self, key, value):
>        if key.lower() == 'location':
>            clientCreator = protocol.ClientCreator(reactor, Redis, db=3)
>            redis = yield clientCreator.connectTCP("localhost", 6379)
>            value = yield redis.get('myproxy:hashcode:%s' % value)
>        proxy.ProxyClient.handleHeader(self,key,value)
>as the handleHeader caller doesn't wait for a deferred to finish, the 
>location header is added after handleResponseEnd is already called and 
>therefore never makes it to the client.
>is there a way to convert the non-blocking redis call to a blocking 
>redis call?
>i know that there's a blocking client library for redis but i'd rather 
>use only txredis if possible.

The proxy isn't written to deal with what you want to do.  Converting 
the non-blocking call into a blocking one won't fix the problem, it'll 
just change it.

If you want to change the headers that pass through the proxy, you 
probably need to hook in at a different place, where an asynchronous 
operation won't mess things up.

More generally, `twisted.web.proxy` isn't really well suited for a 
rewriting proxy.  It's barely capable of functioning as a boring pass- 
through proxy.  If you'd like to make your application simpler, you 
might want to look at ways to make `twisted.web.proxy` more well suited 
to this task.


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